An information recording/reproducing device disclosed in Patent document 1 is known as an example of conventional information recording/reproducing devices of this sort. Here, a description is given with reference to FIGS. 5 and 6 based on this conventional example, with some modifications applied thereto.
FIG. 5 is a diagram showing a portion of the cross-sectional structure of an information recording/reproducing device and the state of ray bundles inside the information recording medium during signal recording, according to the conventional art.
As shown in FIG. 5, P-polarized laser light emitted from a radiation light source such as a semiconductor laser is converged by a collimating lens into parallel light, which then is split into two components by a half mirror. One of the two components split by the half mirror is transmitted through a half-wave plate to become S-polarized light 2a (the state of light described thus far is not shown in FIG. 5, but the proceeding state is shown by broken lines), and is transmitted successively through a spatial modulation element 17 and a convex lens 4′ to become light with weak convergence, which then enters a polarizing beam splitter 5′. The light 2a incident on the polarizing beam splitter 5′ is reflected at a plane of polarization 5a′ of the polarizing beam splitter 5′, and is transmitted through a joint gyrator 18. The light 2a then is converged by the objective lens 7 into a ray bundle 20a that is converged onto a point Pb located before a reflection plane 8S of the information recording medium 8. At the point Pb, the ray bundle 20a moves to the opposite side of an optical axis L.
The joint gyrator 18 is formed by bonding together two optical rotators consisting of two regions that are divided on a plane passing through the optical axis L and perpendicular to the plane of FIG. 5. The upper half region 18a causes the direction of polarization of transmitted light to rotate clockwise by 45°, and the lower half region 18b causes the direction of polarization of transmitted light to rotate counterclockwise by 45°.
The other light 2b of the P-polarized laser light that has been emitted from the radiation light source, has been converged by the collimating lens into parallel light, and then has been split into two components by the half mirror (the state of the light described thus far is not shown in FIG. 5, but the proceeding state is shown by solid lines) is transmitted successively through the plane of polarization 5a′ of the polarizing beam splitter 5′ and the joint gyrator 18 along the optical axis L, and then converged by the objective lens 7 into a ray bundle 20b that is converged onto the point P0 on the reflection plane 8S of the information recording medium 8.
The information recording medium 8 is made up of a transparent layer 8a, a transparent substrate 8c, and a photosensitive layer 8b such as a photopolymer interposed therebetween. The converging point P0 and the converging point Pb are formed approximately symmetrically with respect to the photosensitive layer 8b. 
The spatial modulation element 17 may be constituted, for example, by a ferroelectric liquid crystal panel, and the region thereof through which the light 2a is transmitted is divided into a grid pattern. Each divided portion of the region individually is subjected to a phase shift of π (phase modulation), or to a change in the transmittance to zero (amplitude modulation). This modulation pattern is updated based on an external signal.
As can be seen clearly from FIG. 5, the ray bundle 20a and the ray bundle 20b intersect in the photosensitive layer 8b region of the information recording medium 8. A ray 2B, which passes through a circle 19, of the ray bundle 20b is rotated counterclockwise by 45° with respect to the P-polarized light by the region 18b of the joint gyrator 18, and a ray 2A, which passes through the circle 19, of the ray bundle 20a is rotated clockwise by 45° with respect to the S-polarized light by the region 18a of the joint gyrator 18. As a result, the directions of polarization of the rays 2A and 2B coincide in the circle 19. This relationship also applies to rays passing through a circle located opposite to the circle 19 across the optical axis L. Since the directions of polarization of the ray bundles 20a and 20b thus coincide in the photosensitive layer 8b region, the ray bundles 20a and 20b interfere with each other, thereby forming interference fringes. When the output of the radiation light source such as a semiconductor laser is large, the photosensitive layer 8b is exposed by the ray bundles 20a and 20b, thus forming an exposed pattern 21 (a pattern in which the refractive index is varied in correspondence with the light intensity distribution of the interference fringes). This exposed pattern 21 corresponds to the modulation pattern of the spatial modulation element 17 (i.e., different exposed patterns 21 are recorded depending on the modulation pattern of the spatial modulation element 17).
The information recording medium 8 is attached to a motor, and is rotated by the rotation of that motor. On the surface of the reflection plane 8S, guide grooves (gratings) having a periodicity in the radius direction are formed with an equal pitch along the direction of rotation.
The converging point P0 of the ray bundle 20b is located on the guide grooves, and moves along the guide grooves with the rotation of the information recording medium 8. After being reflected at the reflection plane 8S, the ray bundle 20b is transmitted through the information recording medium 8, and converted into parallel light by the objective lens 7. The light whose direction of polarization has been rotated clockwise by 45° by being transmitted through the region 18a of the joint gyrator 18 in the incoming path is transmitted through the region 18b of the joint gyrator 18 in the outgoing path, and its direction of polarization thus is returned to the original direction. Similarly, the light whose direction of polarization has been rotated counterclockwise by 45° by being transmitted through the region 18b of the joint gyrator 18 in the incoming path is transmitted through region 18a of the joint gyrator 18 in the outgoing path, and its direction of polarization thus is returned to the original direction. As a result, the return light from the ray bundle 20b is restored to P-polarized light by being transmitted through the joint gyrator 18. After being transmitted through the plane of polarization 5a′ of the polarizing beam splitter 5′, the P-polarized light is guided to the photodetector side by a splitting means such as a hologram (this is not shown in FIG. 5). Then, a focus error signal for the reflection plane 8S and a tracking error signal for the guide grooves are generated by a detection signal from the photodetector, and the objective lens 7 is driven based on these signals such that the converging point of the ray bundle 20b is controlled so as to be located on the guide grooves on the reflection plane 8S.
FIG. 6 is a diagram showing a portion of the cross-sectional structure of an information recording/reproducing device and the state of ray bundles inside the information recording medium during signal reproduction, according to the conventional art. FIG. 6 is different from FIG. 5 in the modulation pattern of the spatial modulation element 17.
As shown in FIG. 6, P-polarized laser light emitted from the radiation light source is converged by the collimating lens into parallel light, which then is split into two components by the half mirror. Light 2b′, which is one of the two components split by the half mirror (the state of the light described thus far is not shown in FIG. 6, but the proceeding state is shown by solid lines) is transmitted successively through the plane of polarization 5a′ of the polarizing beam splitter 5′ and the joint gyrator 18 along the optical axis L, and its directions of polarization are rotated clockwise and counterclockwise by 45° respectively by the regions 18a and 18b of the joint gyrator 18. Thereafter, the light 2b′ is converged by the objective lens 7 onto the point P0 located on the reflection plane 8S of the information recording medium 8. The ray (reflected light) 2B′ reflected at the reflection plane 8S propagates through the circle 19 in the exposed pattern 21, and, thus, becomes a ray bundle 20a′, which produces diffracted light 2A. After being converged onto the point Pb, the diffracted light 2A′ moves to the opposite side of the optical axis L. The diffracted light 2A′ is converted into light with weak divergence by the objective lens 7, and its polarization state is changed by the joint gyrator 18. Since the polarization state of the diffracted light 2A′ is the same as that of the reflected light 2B′, and the P-polarized light is transmitted through the region 18a of the joint gyrator 18 in the incoming path, the diffracted light 2A′ is rotated clockwise by 45° with respect to the P-polarized light. On the other hand, the diffracted light produced by the propagation through a circle located opposite to the circle 19 across the optical axis L is rotated counterclockwise by 45° with respect to the P-polarized light. When these diffracted light rays have been transmitted through the joint gyrator 18, their directions of polarization are rotated clockwise and counterclockwise by 45° respectively by the regions 18a and 18b of the joint gyrator 18. As a result, the ray bundle 20a′ is converted into an S-polarized ray bundle. Accordingly, the ray bundle 20a′ is reflected at the plane of polarization 5a′ of the polarizing beam splitter 5′, converted into parallel light by the convex lens 4, and then is transmitted through the spatial modulation element 17. As described above, the spatial modulation element 17 includes a ferroelectric liquid crystal panel or the like, and during signal reproduction, allows the transmitted light to be transmitted therethrough, without applying modulation, based on an external signal. The light 2a′, which has been transmitted through the spatial modulation element 17, is in accordance with the recording signal of the exposed pattern 21, so that the modulation pattern of the spatial modulation element 17 during signal recording is reproduced as a light intensity distribution pattern of the light 2a′. The transmitted light 2a′ is branched from the incoming optical path by a hologram, a half mirror, or the like, and converged by a collimating lens so as to be guided to the photodetector. Then, a signal light is detected by photodetection cells in the form of grids corresponding to the pattern of division of the spatial modulation element 17, and, thereby, the recording signal is reproduced.
On the other hand, the component 20b′, which has not been diffracted by propagating through the exposed pattern 21 region, is transmitted through the information recording medium 8, and then converted into parallel light by the objective lens 7. The light whose direction of polarization has been rotated clockwise by 45° by being transmitted through the region 18a of the joint gyrator 18 in the incoming path is transmitted through the region 18b of the joint gyrator 18 in the outgoing path, and its direction of polarization thus is returned to the original direction. Similarly, the light whose direction of polarization has been rotated counterclockwise by 45° by being transmitted through the region 18b of the joint gyrator 18 in the incoming path is transmitted through region 18a of the joint gyrator 18 in the outgoing path, and its direction of polarization thus is returned to the original direction. As a result, the reflected ray bundle 20b′ is restored to P-polarized light by being transmitted through the joint gyrator 18. After being transmitted through the plane of polarization 5a′ of the polarizing beam splitter 5′, the P-polarized light is guided to the photodetector side by a splitting means such as a hologram. Then, a focus error signal for the reflection plane 8S and a tracking error signal for the guide grooves are generated by a detection signal from the photodetector, and the objective lens 7 is driven based on these signals such that the converging point P0 of the convergent light is controlled so as to be located on the guide grooves on the reflection plane 8S.
Patent document 1: JP H11-311938A